We have theoretically studied acoustic phonon spectra and phonon propagation in rectangular nanowires embedded within elastically dissimilar materials. As example systems, we have considered GaN nanowires with AlN and plastic barrier layers. It has been established that the acoustically mismatched barriers dramatically influence the quantized phonon spectrum of the nanowires. The barriers with lower sound velocity compress the phonon energy spectrum and reduce the phonon group velocities in the nanowire. The barriers with higher sound velocity have an opposite effect. The physical origin of this effect is related to redistribution of the elastic deformations in the acoustically mismatched nanowires. In the case of the “acoustically slow” barriers, the elastic deformation waves are squeezed in the barrier layer. The effect predicted for the nanowires embedded with elastically dissimilar materials could be used for reengineering phonon spectrum in nanostructures.

Acoustic-phonon propagation in rectangular semiconductor nanowires with elastically dissimilar barriers

Acoustic phonon engineering in coated cylindrical nanowires

With its properties of bandgap tunability, low cost, and substrate compatibility, colloidal quantum dots (CQDs) are becoming promising materials for optoelectronic applications. Additionally, solution-processed organic, inorganic, and hybrid ligand-exchange technologies have been widely used in PbS CQDs solar cells, and currently the maximum certified power conversion efficiency of 9.9% has been reported by passivation treatment of molecular iodine. Presently, there are still some challenges, and the basic physical mechanism of charge carriers in CQDs-based solar cells is not clear. Electrochemical impedance spectroscopy is a monitoring technology for current by changing the frequency of applied alternating current voltage, and it provides an insight into its electrical properties that cannot be measured by direct current testing facilities. In this work, we used EIS to analyze the recombination resistance, carrier lifetime, capacitance, and conductivity of two typical PbS CQD solar cells Au/PbS-TBAl/ZnO/ITO and Au/PbS-EDT/PbS-TBAl/ZnO/ITO, in this way, to better understand the charge carriers conduction mechanism behind in PbS CQD solar cells, and it provides a guide to design high-performance quantum-dots solar cells.

Charge Carrier Conduction Mechanism in PbS Quantum Dot Solar Cells: Electrochemical Impedance Spectroscopy Study.

Organic photomultiplication photodetectors have attracted considerable research interest due to their extremely high external quantum efficiency and corresponding high detectivity. Significant progress has been made in the aspects of their structural design and performance improvement in the past few years. There are two types of organic photomultiplication photodetectors, which are made of organic small molecular compounds and polymers. In this paper, the research progress in each type of organic photomultiplication photodetectors based on the trap assisted carrier tunneling effect is reviewed in detail. In addition, other mechanisms for the photomultiplication processes in organic devices are introduced. Finally, the paper is summarized and the prospects of future research into organic photomultiplication photodetectors are discussed.

https://www.mdpi.com/2079-4991/8/9/713/pdf

Research Progress in Organic Photomultiplication Photodetectors.

The end-to-end P–N heterojunction nanowire arrays combined organic (poly[1,4-bis(pyrrol-2-yl)benzene], BPB) and inorganic (CdS) molecules have been successfully designed and fabricated. The electrical properties of P–N heterojunctions of organic–inorganic nanowire arrays were investigated. The diode nature and rectifying feature of P–N heterojunction nanowire arrays were observed. The rectification ratio of the diode increased from 29.9 to 129.7 as the illumination intensity increased. The material exhibits a new property, which is an improvement in the integration of the physical and chemical properties of the two independent components.

Synthesis and characterization of axial heterojunction inorganic–organic semiconductor nanowire arrays

Different emission mechanism of organic–inorganic heterostructure device

The charge carriers conduction mechanism of single-layer device ITO/Alq 3 (90 nm)/Al has been studied by impedance spectroscopy. According to the frequency-dependent response and its equivalent circuit model，the device can be interpreted in terms of the equivalent circuit model of a contact resistance R s in series with a parallel combination of a capacitance C p and a resistance R p , and the values of R p , C p and R s can be found from experimental data. Our experimental results confirmed that the charge carriers conduction mechanism of ITO/Alq 3 (90 nm)/Al is trap-charge limited current with exponential distribution, and the dielectric relaxation time of the material decreases dramatically with increasing applied bias voltages.

Dynamic electrical characteristics of organic light-emitting diodes

Due to the excellent characteristics in field-effect transistors and its high absorption coefficient in the visible region, pentacene has been widely used in phototransistors. The channel length of the vertical transistor could be designed very short to nanometers. In this way, the device performances and its working frequency can be effectively improved, and the energy consumption can be reduced simultaneously. In this paper, we fabricated a kind of low-voltage pentacene photodetector ITO(S)/Pentacene/Al(G)/Pentacene/Au(D), basing on the vertical transistor configuration. The threshold voltage and “on/off” current ratio were -0.9 V and 104 at a low working-voltage of -3 V, respectively. The pentacene photodetector ITO/Pentacene(80nm)/Al(15nm)/Pentacene (80nm)/Au exhibited good p-type transistor behavior and low-voltage-controlling performance. The photosensitivity and responsivity varied with incident monochromatic light from 350 nm to 750 nm, and the photosensitivity peak of 308 was obtained at 350 nm with a responsivity of 219 mA/W, which is even higher than the standard Si-based photodetector under 350 nm incident light. Therefore, it provides an easy way to get a high sensitivity all-organic photodetector working at low voltages.

Low-voltage pentacene photodetector based on a vertical transistor configuration

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To Improve the Efficiency of Bulk Heterojunction Organic Solar Cells by Incorporating CdSe/ZnS Quantum Dots

Pure green and narrow bandwidth emission from organic bilayer electroluminescent devices were presented by using Tb\+\{3+\}∶(SA)\-3 as hole\|transporting layer and emissive layer, and high fluorescent material Alq\-3 acting as electron\|transporting layer. It proves that the emission comes from carriers\' recombination after their tunneling through the inner interface (Tb\+\{3+\}∶(SA)\-3/Alq\-3) in the bilayer ITO/Tb\+\{3+\}∶(SA)\-3(40nm)/Alq\-3(55?nm)/Al diode. The spectrum of the bilayer devices varies with the thickness of the electron\|transporting layer. Keeping the thickness of Tb\+\{3+\}∶(SA)\-3 layer constant, there is no emission from Alq\-3 layer when Alq\-3 layer reduces to 12?nm; meanwhile, pure green and narrow bandwidth emission can be observed. Thus, it provides a simple way to obtain high luminance and long\|term stability by reducing electron\| or hole\|transporting layer, especially, by using a lanthanide complex for emissive layer.

Yang Sheng-Yi

Papers

Different emission mechanism of organic–inorganic heterostructure device

Dynamic electrical characteristics of organic light-emitting diodes

Low-voltage pentacene photodetector based on a vertical transistor configuration

To Improve the Efficiency of Bulk Heterojunction Organic Solar Cells by Incorporating CdSe/ZnS Quantum Dots

Pure green and narrow bandwidth electroluminescence from organic light-emitting diodes